CD200 and COVID-19: Finally, a total of eleven differential factors related to COVID-19 disease severity were identified, including: TRAIL R1, IGFBP-1, IGFBP-4, VCAM-1, sFRP-3, FABP2, Transferrin, GDF15, IL-1F7 (also known as IL-37), IL-5Rα, and CD200.